Recently, small information communications equipments such as mobile phones, etc. are getting high performance and high functions, and web terminals, music players, etc. are required to be continuously usable for a long period of time. These small information communications equipments and electronic equipments use secondary batteries such as lithium ion batteries, etc. as power sources.
Methods for charging secondary batteries include contact charging systems and non-contact charging systems. The contact charging systems are systems of charging electricity by bringing an electrode of a power-receiving device into direct contact with an electrode of a power-supplying device, and have conventionally been used widely because of simple structures, though they need a connection plug adapted to the electrode of power-receiving device and a charging device. The non-contact charging systems comprise a power-supplying device and a power-receiving device each having a coil, conducting power transmission by electromagnetic induction. Because the non-contact charging systems do not need electrodes for the direct contact of a power-supplying device to a power-receiving device, different power-receiving devices can be charged by the same power-supplying device. Also, because the non-contact charging systems do not use corrodible electrodes, they can stably supply and receive electric energy.
In a non-contact charging apparatus, magnetic flux generated by a primary coil acts to generate an electromotive force in a secondary coil via a case of a power-supplying device and a case of a power-receiving device. To improve power transmission efficiency, a coil yoke for preventing the leakage of magnetic flux generated from a coil while charging is disposed on each rear surface of a primary coil and a secondary coil. The leaked magnetic flux flowing through other parts and metal members would cause heat generation by eddy current, and a coil yoke acts as a magnetic shield preventing it. For instance, JP 11-176677 A discloses a non-contact charging apparatus comprising a ferrite core between a secondary coil and circuit parts.
In the case of coreless primary and secondary coils, they should overlap each other in as wide an area as possible to obtain high power transmission efficiency. Accordingly, their center axes should be aligned. For instance, JP 11-176677 A describes that if the center axes of both coils were deviated by a radius of each coil, transmitted power would be zero. For the precise positioning of a power-receiving device to a power-supplying device, there is a system in which a case of a power-supplying device is fitted in a case of a power-receiving device. In this system, however, a power-supplying device should have a power-supplying surface adapted to each power-receiving device, failing to have a common power-supplying device for various power-receiving devices.
JP 2008-301553 A discloses an apparatus for charging a chargeable equipment such as a mobile phone, etc. in a non-contact manner, in which a cradle comprising a primary coil in its case comprises a substantially flat mounting plate having an area enough to mount the chargeable equipment with margin, the mounting plate having a mirror portion including a mark indicating an adaptable range corresponding to a center of the primary coil, the chargeable equipment being positioned on the mounting plate with a center mark reflected by the mirror portion, such that the center mark indicating a center position of a secondary coil in the chargeable equipment is aligned with the adaptable range mark of the mounting plate. In this apparatus, however, each chargeable equipment should be provided with a center mark in advance, undesirable from the viewpoint of design, and the aligning of both marks cannot be conducted in the dark.
JP 2009-159677 A discloses a non-contact charging adapter comprising mounting means constituted by a permanent magnet disposed on a rear side of a power-supplying surface of a power-supplying device, and a permanent magnet disposed on a rear side of a charging surface of a charging device, for positioning a primary coil to a secondary coil by magnetic attraction such that their center axes are aligned. Each mounting means comprises two thin, L-shaped permanent magnets combined such that they annually extend along a periphery of the device. Because each device comprises an annular permanent magnet disposed outside the coil, the arrangement of a coil yoke on a rear side of the coil makes magnetic flux generated from the permanent magnet easily flowable through the coil yoke disposed inside, so that a portion of the coil yoke adjacent to the permanent magnet tends to be magnetically saturated. Because the magnetically saturated portion of the coil yoke has drastically decreased permeability, it fails to act as a yoke sufficiently, resulting in low power transmission efficiency. In addition, the thin, L-shaped permanent magnet is difficult to produce by a rare-earth magnet, and easily broken during handling. If the thin, L-shaped permanent magnet were formed, for instance, by a rubber magnet, sufficient magnetic power would not be obtained, resulting in poor positioning accuracy.